You can check out what tracks are hidden on your account here.
To delete tracks, click to view hidden tracks and hover your mouse over the track for the trash icon to appear.

Please note that it can sometimes take a moment for the track deletion to be fully processed on our backend. If you just deleted some tracks to make space but are unable to upload more or if the system still suggests you are over quota, please do the following:

Log out of your SoundCloud account, clear your browser's cache, restart it, then try again. This will ensure your browser doesn't try to fetch old data from its local cache.
Also, please note that it can take a few minutes for our deletion systems to fully process your track deletion, so please give this a moment.

Cheers
Mathis

Help other community members find useful content by clicking the like button and selecting the best answer to your question.